This invention relates to a color cathode-ray tube and, more particularly, to a color cathode-ray tube incorporating a shadow mask having slit apertures.
Generally, in a color cathode-ray tube incorporating a shadow mask having slit apertures, a phosphor screen having groups of red, green and blue phosphor stripes is formed on the inner surface of the face plate. In the neck of the cathode-ray tube, in-line type electron guns are received. Three electron beams are emitted from the electron guns to pass through the slit apertures and land on the phosphor stripes while they are deflected by a deflection device provided around a funnel.
In such a color cathode-ray tube, the electron beams are deflected and the shadow mask is scanned by the electron beams, so that the electron beams impinge on and heat the shadow mask when the electron beams are not directed to the slit apertures. Generally, the amount of electron beams passing through the slit apertures is substantially less than one-third of the total emitted electron beams. The energy of the rest of the electron beams is consumed as the electron beams strike and heat the shadow mask. For this reason, the shadow mask is sometimes heated up to as high a temperature as 80.degree. C. The shadow mask is usually made of a material mainly composed of iron which has a relative high coefficient of thermal expansion and has a thickness of 0.1 to 0.3 mm, and its peripheral edge is reinforced by a mask frame about 1 mm in thickness and having high mechanical strength. When the shadow mask is heated by the electron beams and expanded, it is deformed into a dome-like shape, thus changing the distance between it and phosphor screen (the distance being referred to as Q value). When the Q value exceeds a predetermined value, the electron beams no longer accurately land on the corresponding phosphor stripes, that is, mislanding occurs, thus deteriorating the color purity. In order to prevent the mislanding, the mask frame is fixed by a bimetal member to the face plate inner surface, as disclosed in U.S. Pat. No. 3,803,436. When the shadow mask is heated beyond a predetermined temperature so that it is expanded, its heat is transferred to the bimetal member, causing deformation thereof such as to hold the mislanding within a permissible value. In this case, however, the heat of the shadow mask is transferred to the bimetal member through the mask frame which has high heat capacity. Therefore, it is liable that the shadow mask will undergo thermal expansion into a dome-like shape before the deformation of the bimetal occurs. In this case, temporary mislanding would occur.
Generally, the deformation or doming of the shadow mask depends on the curvature of the shadow mask. Generally, the smaller the curvature of the shadow mask, the greater the deformation or doming of the shadow mask due to heating thereof. In other words, the greater the radius of curvature of the shadow mask, i.e., the flatter the shadow mask, the greater the deformation to cause the color purity deterioration. More simply, the color purity is more liable to be deteriorated by increasing the radius of curvature of the shadow mask and thus making it flatter. For this reason, in the conventional color cathode-ray tube, the radius of curvature of the shadow mask is made relatively small so that the shadow mask is curved relatively greatly while the face plate is also curved relatively greatly so that the phosphor screen itself is curved relatively greatly. With the color cathode-ray tube where the face plate is curved in this way, it is rather difficult to accurately monitor the image or pattern. This problem is prominent for a color cathode ray tube with a greater screen size. That is, with the radius of curvature fixed the doming is prominent with a shadow mask of larger size than with a shadow mask of a smaller size to an extent that it cannot be disregarded.
Further, if it is intended to make the inner surface of the face plate of the color cathode-ray tube flatter so that the front shape thereof has substantially a rectangular shape, the following problem arises in addition to the phenomenon of doming described above. Where the shadow mask and face plate inner surface have a relatively small radius of curvature and curved relatively greatly, the phosphor screen has a barrel-like shape as shown in FIG. 1. In this phosphor screen, phosphor stripes in end regions on the horizontal axis X--X are curved with a relatively small radius of curvature having the center of curvature in the phosphor screen. In other words, the distance between the corners of the phosphor screen along the horizontal axis X--X is smaller than the width H of the phosphor screen on the horizontal axis X--X, that is, there is a difference .DELTA.H between them. In a color cathode-ray tube where the face plate inner surface is made flatter so that it has substantially a rectangular front shape, the difference .DELTA.H between the dimensions H and h is certainly reduced. In this case, the phosphor stripes in the pheripheral regions of the phosphor screen remotest from the vertical axis Y--Y are curved with a relatively large radius r of curvature. However, since the front portion of the face plate has substantially rectangular shape, the phosphor screen which is curved only slightly would seem more dissatisfactory in appearance than the conventional cathode-ray tube.